DAOS-16566 test: Update server/multiengine_persocket.py (#15127)

[phender]

Update the server/multiengine_persocket.py test to use test harness methods to ensure compatability with future test harness updates. Also add a test_mode option to allow verification of all but the PMem reconfiguration in CI.

Skip-unit-tests: true
Skip-fault-injection-test: true

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[github-actions]

Ticket title is 'ftest: Update server/multiengine_persocket.py to use test harness methods'
Status is 'Awaiting backport'
Labels: 'manual_test,testp2,request_for_2.6.2'
https://daosio.atlassian.net/browse/DAOS-16566